Segment polarity genes were originally discovered as mutations in flies that change the pattern of body segment structures. Mutations in these genes cause animals to develop changed patterns on the surfaces of body segments; the changes affecting the pattern along the head to tail axis. Among the genes in this class are hedgehog, which encodes a secreted protein (HH), and patched, which encodes a protein structurally similar to transporter proteins, having twelve transmembrane domains (ptc), with two conserved glycosylation signals.
The hedgehog gene of flies has at least three vertebrate relatives—Sonic hedgehog (Shh); Indian hedgehog (Ihh), and Desert hedgehog (Dhh). Shh is expressed in a group of cells, at the posterior of each developing limb bud, that have an important role in signaling polarity to the developing limb. The Shh protein product, SHH, is a critical trigger of posterior limb development, and is also involved in polarizing the neural tube and somites along the dorsal ventral axis. Based on genetic experiments in flies, patched and hedgehog have antagonistic effects in development. The patched gene product, ptc, is widely expressed in fetal and adult tissues, and plays an important role in regulation of development. Ptc downregulates transcription of itself, members of the transforming growth factor and Wnt gene families, and possibly other genes. Among other activities, HH upregulates expression of patched and other genes that are negatively regulated by patched.
It is of interest that many genes involved in the regulation of growth and control of cellular signaling are also involved in oncogenesis. Such genes may be oncogenes, which are typically upregulated in tumor cells, or tumor suppressor genes, which are down-regulated or absent in tumor cells. Malignancies may arise when a tumor suppressor is lost and/or an oncogene is inappropriately activated. Familial predisposition to cancer may occur when there is a mutation, such as loss of an allele encoding a suppressor gene, present in the germline DNA of an individual.
The most common form of cancer in the United States is basal cell carcinoma of the skin. While sporadic cases are very common, there are also familial syndromes, such as the basal cell nevus syndrome (BCNS). The familial syndrome has many features indicative of abnormal embryonic development, indicating that the mutated gene also plays an important role in development of the embryo. A loss of heterozygosity of chromosome 9q alleles in both familial and sporadic carcinomas suggests that a tumor suppressor gene is present in this region. The high incidence of skin cancer makes the identification of this putative tumor suppressor gene of great interest for diagnosis, therapy, and drug screening.
Relevant Literature
Descriptions of patched, by itself or its role with hedgehog may be found in Hooper and Scott (1989) Cell 59-.751-765; and Nakano et al. (1989) Nature 341-.508-513. Both of these references also describe the sequence for Drosophila patched. Discussions of the role of hedgehog include Riddle et al. (1993) Cell 75-.1401-1416-, Echelard et al. (1993) Cell 75:1417-1430-Krauss et al. (1993) Cell 75:1431-1444 (1993); Tabata and Kornberg (1994) 76:89-102; Heemskerk and DiNardo (1994) Cell 76:449-460; and Roelink et al. (1994) Cell 76:-761-775.
Mapping of deleted regions on chromosome 9 in skin cancers is described in Habuchi et al. (1995) Oncogene 11: 1671-1674, Quinn et al. (1994) Genes Chromosome Cancer 11:222-225; Quinn et al. (1994) J. Invest. Dermatol. 102:300-303; and Wicking et al. (1994) Genomics 22:505-511.
Gorlin (1987) Medicine 66:98-113 reviews nevoid basal cell carcinoma syndrome. The syndrome shows autosomal dominant inheritance with probably complete penetrance. About 60% of the cases represent new mutations. Developmental abnormalities found with this syndrome include rib and craniofacial abnormalities, polydactyly, syndactyly and spina bifida. Tumors found with the syndrome include basal cell carcinomas, fibromas of the ovaries and heart, cysts of the skin, jaws and mesentery, meningiomas and medulloblastomas.